Why can't I change the weights of subdivided bones?
It looks like some of the vertices of the biceps are influenced by several bones, if you select one vertex of this zone and open the N panel (on the right of your 3D View) &gt; Item tab &gt; Vertex Weights, you'll see a series of vertex groups that the vertex should not be part of.
Also, if you select your armature and go in the Properties panel &gt; Object Data &gt; Skeleton &gt; Layers, you have a lot of hidden bones that have their Deform option activated and that will influence this part of the body.
For example it looks like the wrist is the parent of some upper arm bones, therefore when you rotate the wrist it deforms the upper arm mesh. It makes no sense, you should probably correct these bones relationships. This correction should fix the problem.

But if you want to entirely redo the weight painting you can select the part of the mesh you want to rework, go in the Properties panel &gt; Object Data &gt; Vertex Groups and on the left of the Vertex Groups list, click on the down arrow icon and choose Remove From All, which will de-assign these vertices from all groups.
Now you can work in Weight Paint mode and give to this part the influence that each bone is supposed to have.
Instead of subdividing your forearm, you could choose to display your bones as B-bones instead of Stick (Properties panel &gt; Object Data &gt; Display As). Then choose the bone you want to segment and in the Properties panel &gt; Bone &gt; Bendy Bones, choose the amount of Segments you want for this bone. Then give it a constraint Copy Rotation that you put above the IK Constraint, and choose the hand as Target, Space &gt; Pose Space / Pose Space. The bone flexibility is set in the Properties panel &gt; Bone &gt; Bendy Bones &gt; Easing.